In the prior art using CEC (Consumer Electronics Control) messages of HDMI (High-Definition Multimedia Interface), it has been known to transmit the CEC messages by wire (e.g., See Patent Document 1). The standard of HDMI is established in Non-Patent Document 1.
Now describe the transmission method of CEC messages in which two devices with an HDMI interface (an Source device and a Sink device) are connected to each other by wire using an HDMI cable and a CEC message is sent from one device to the other. A CEC message to be sent from a sender device (an Source device or a Sink device) is transmitted to a receiver device (a Sink device or an Source device) with the CEC message being contained in one frame.
FIG. 9 is a diagram showing an exemplary format of a CEC frame according to a prior art example and an embodiment of the present invention, and FIG. 10 is a diagram showing an exemplary format of a header block/data block according to a prior art example and an embodiment of the present invention. A CEC frame 1000 is composed of a start bit 1001, a header block 1002, and 0 or more data block(s) 1003. The start bit 1001 is a bit indicating the beginning of the CEC frame, and outputting a predetermined pulse waveform to a transmission path. The header block 1002 and the data block(s) 1003 have the same configuration with each other, and are composed of 8 information bits 1101, an EOM (End of Message) bit 1102, and a ACK bit 1103.
The information bit 1101 of the header block 1002 contains a logical address of the sender device (4 bits), and a logical address of the receiver device (4 bits). The information bit 1101 of the data block 1003 contains a code indicating an operation on the receiver device, etc. The EOM bit 1102 is set to “1” when the data block is the last data block of the CEC frame, and is set to “0” otherwise. The ACK bit 1103 is a bit used to determine whether or not the header block and the data block have been properly received by the receiver device, and is set to “1” by the sender device. When properly receiving the header block and the data block, the receiver device outputs “0” to ACK bits 1103.
In the header block and the data block, pulse waveforms indicating bit values “0” and “1” are continuously outputted to the transmission path. The receiver device detects the start bit 1001, and then samples pulse waveforms indicating the header block 1002 and the data blocks 1003 at predetermined timing, thus receives data.
Further, even in the case that a plurality of devices with an HDMI interface are connected to each other, since CEC lines are of bus connections, the sender device and the receiver device can operate in the same manner.    Patent Document 1: Japanese Patent laid-open Publication No. 2005-524926.    Non-Patent Document 1: High-Definition Multimedia Interface Specification, Version 1.1, HDMI Licensing, LLC, California in U.S.A., May 20, 2004.